The present invention relates to molded articles based on pyrogenically produced titanium dioxide, the method for making the molded articles from pyrogenically produced titanium dioxide, and their use as catalyst-carriers or catalysts.
Pyrogenically produced titanium dioxide is obtained by high-temperature hydrolysis of vaporizable titanium compound, as a rule titanium tetrachloride, in an oxyhydrogen flame. This procedure is described in German patent 870,242 (1953). The resulting products predominantly possess the structure of anatase. They are hydrophilic, highly pure and are of extremely finely divided particulate form. The primary particles (DIN 53206) when viewed under the electron microscope exhibit spherical shapes and diameters of 10-100 nm. A commercial product has an average particle size of 30 nm. There are no defined agglomerates (DIN 53206). The particle surfaces are smooth and pore-free. There is only an external, easily accessible surface on particles. The specific BET surface depends on the conditions of production and can range between 20 and 100 m.sup.2 g The above mentioned commercial product has a specific surface of 50.+-.15 m.sup.2 g.
Because of the high purity, the high specific surface and the lack of pores, pyrogenically produced titanium dioxide is used as the titanium dioxide component or as the carrier in catalytic systems (v. Rives-Arnau, G. Munuera, Appl. Surface Sci. 6 [1980]122; N. K. Nag, T. Fransen, P. Mars, J. Cat. 68, 77 [1981]; F. Solymosi, A. Erdoehelyi, M. Kocsis, J. Chem. Soc. Faraday Trans. 1, 77, 1003 [1981]; D. G. Mustard, C. H. Bartholomew, J. Cat. 67, 186 [1981]; M. A. vannice, R. L. Garten, J. Cat. 63, 255 [1980]and M. A. Vannice, R. L. Garten, J. Cat. 66, 242 [1980]).
However the above cited literature relates only to pulverulent catalytic systems. If now pyrogenically produced titanium dioxide is to be used on an industrial scale in catalytic systems, the powdery, product appropriately must be converted into pressed bodies while substantially retaining its specific surface and the easy accessibility.
Pyrogenically produced titanium dioxide being of particularly finely divided particulate nature, difficulties are encountered when pressing the powder into catalyst-carriers.
The German OLS 31 32 674 describes a procedure for making pressed articles from pyrogenically produced titanium dioxide and employing a silica gel as the binder. Auxilliary pressing aids used are ethane diol, glycerin, erythrite, pentite or hexite. This procedure incurs the drawback that when the pressed articles are made industrially on a large scale, undesired "topping-off" takes place in the pressed articles, that is, an upper layer detaches off the pressed articles.
Moreover, in the prior cut methods besides TiO.sub.2, the pressed articles also contain substantial amounts of SiO.sub.2 due to the nature of the binder and produces an affect on the catalysis.
The German OLS 32 17 751 discloses the pressing of pyrogenically produced titanium dioxide, wherein organic acids or acidically reacting salts are employed as intermediate binders. In addition, graphite is used as a pressing aid.
This procedure suffers from the drawback that a high annealing temperature exceeding 700.degree. C. is required to remove the graphite.
This is attended by the danger that a phase change may take place in the titanium dioxide. If so, the desired anatase will be converted into the undesired rutile form.